Often, it is necessary in an industrial or other process to inject a measured quantity of a flowable material into a further stream of material or a vessel. Metering pumps have been developed for this purpose and may be either electrically or hydraulically actuated.
Conventionally, an electromagnetic metering pump utilizes a linear solenoid which is provided electrical pulses to move a diaphragm mechanically linked to an armature of the solenoid. As the solenoid is energized and deenergized, the armature and the diaphragm are reciprocated in suction and discharge strokes over a range of positions. Referring to FIG. 1, during each suction stroke, liquid is drawn upwardly through a first fitting 51 past a first check valve 53 and enters a diaphragm recess 55. A second check valve 57 is closed during the suction stroke. During each discharge stroke, the first check valve 53 is closed and the second check valve 57 is opened, thereby allowing the liquid to travel upwardly past the second check valve 57 and a fitting 59 and outwardly of the pump 21.
A stroke length adjustment member sets the stroke length of the armature 31 (i.e., the distance the armature travels during each suction and discharge stroke). As shown in FIG. 1, the stroke length adjustment member is conventionally a combination of a screw 40 and a knob 42. The armature 31 rests against an end of the screw 40 at the end of each suction stroke. The position of the end of the screw 40, and thus the stroke length, can be adjusted by manually rotating the knob 42 in either a first or second direction.
When the pump is not operating, however, the screw 40 can be rotated to shorten the stroke length only when the armature 31 is spaced from the end of the screw 40, i.e., when the armature is not at the end of a suction stroke. This is because the screw 40 is not capable of providing the required mechanical force to change the stroke length when the armature 31 is in contact with the end of the screw 40.